Methods for improving performance of holographic glasses

ABSTRACT

The improvement of the performance of holographic glasses with recorded holograms as measured by a figure of merit of the holographic glasses is disclosed. The improvement in the figure of merit of the holographic glasses is obtained at least in part with the addition of arsenic in the formation of the holographic glasses. The presence of arsenic increases the figure of merit as measured at a wavelength of interest of a holographic glass with a recorded hologram as compared to a holographic glass with a recorded hologram that does not contain arsenic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/111,090, filed Apr. 28, 2008, which claims benefit under 35 U.S.C.§119(e) of provisional U.S. patent application No. 60/914,052, filedApr. 26, 2007, the entirety of which is incorporated herein byreference.

BACKGROUND

Glasses that are capable of producing changes in color are sometimesreferred to as “polychromatic” glasses. Glasses that are capable ofproducing refractive index modulation upon illumination by light,followed by thermal treatment, are sometimes referred to as“photorefractive” or “photo-thermal-refractive” (PTR) glasses. Suchglasses are well known, and may be referred to generally herein as“holographic” glasses. Examples of such holographic glasses have beendisclosed in, for example, U.S. Pat. No. 4,017,318 (“Pierson”), U.S.Pat. No. 4,514,053 (“Borrelli”), U.S. Pat. No. 6,586,141 (“Efimov”), andSoviet patent SU 1,780,429 (“Borgman”). The entire disclosures of eachof the foregoing patents are incorporated herein by reference.

A characteristic of a holographic glass is the optical performance, in avery general sense, of the glass at a certain frequency or wavelength oflight, or over a range of frequencies or wavelengths of light, beforeand after a hologram has been recorded in the glass. The performance ofsuch a holographic glass may be measured by a so-called “figure ofmerit” (FOM). For better optical performance, it may be desirable toincrease the FOM of holographic glasses.

SUMMARY

It has been discovered that the introduction of arsenic into aholographic glass composition increases the performance of theholographic glass, at least in the visible and ultraviolet spectra. Suchperformance improvement may be demonstrated by an increase in the figureof merit (FOM) of the glass. FOM may be measured as a ratio of thechange in the index of refraction of the holographic glass to the lightloss of the glass at a predetermined test light wavelength after a Bragggrating is holographically recorded in the glass. Arsenic may beintroduced into the glass composition during the manufacturing processin the form of an arsenic compound, such as As₂O₃, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of absolute loss coefficient measured in certainholographic glasses.

FIGS. 2A and 2B are illustrative examples of raw material schedules forholographic glass that contain an arsenic compound.

FIG. 3 is an illustrative example of a range of components of aholographic glass produced from illustrative methods and the rawmaterials illustrated in FIGS. 2A and 2B.

FIG. 4 is a plot of the figures of merit for an example holographicglass made with an arsenic compound, at various wavelengths of interest.

FIG. 5 is a listing of the figures of merit for an example holographicglass made with an arsenic compound, at various wavelengths of interest.

FIG. 6 is a plot of photo-induced losses in a holographic glass madewith an arsenic compound at a wavelength of interest.

FIG. 7 is a plot of absorption in a holographic glass made with anarsenic compound, before and after final thermal treatment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIGS. 2A and 2B provide example tables of raw materials that may be usedin the formation of such holographic glasses. The tables provide listsof raw materials, along with the percentage by weight of each, for twoexample formulations. In both formulations, arsenic may be supplied inthe form of As₂O₃. In the first example, provided in FIG. 2A, the As₂O₃may make up 0.1106% of the total combined weight of all the inputingredients. In the second example, provided in FIG. 2B, the As₂O₃ maymake up 0.1146% of the total combined weight of all the inputingredients. Though the example formulations provided herein introducearsenic in the form of As₂O₃, it should be understood that other oxidesof arsenic could be used, and that the arsenic could be supplied inelemental form. It should also be understood that the specificcombinations of ingredients, as well as the relative proportions of theingredients in each formulation, are provided for illustrative purposesonly. Any desirable combination of ingredients may be used, incombination with arsenic or an arsenic compound, in any desirableproportions, to form a holographic glass containing arsenic.

Holographic glasses containing arsenic may be produced using any of anumber of known methods. Such methods are well known to those skilled inthe art, and need not be described herein in detail. However, a summarymethod for forming a holographic glass is provided for context.

According to one such method, a batch of raw materials, such as thoselisted in FIG. 2A or 2B, for example, may be combined and mixed in ablender that is suitable for such purposes. An initial volume of theblended raw materials may be loaded (or “charged”) into a crucible,which may be made of quartz, alumina, or platinum, for example.

The crucible containing the blended raw materials may be placed into afurnace. The temperature of the furnace may be raised gradually,according to a desired schedule, to a desired maximum temperature.Additional volumes of the blended raw materials may be added to thecrucible as the temperature rises, in an effort to keep the cruciblefilled to a certain level. The contents of the crucible may form aso-called glass “melt.”

The glass melt may be “fined” by maintaining the melt at a desiredtemperature and stirring it. The melt may be cooled in the furnace to adesired casting temperature. The cooled melt may be cast directly fromthe furnace or removed from the furnace and cast out according to aknown procedure.

The glass may be annealed by maintaining the glass near the glasstransition temperature for a period of several hours, and then graduallycooling the glass to room temperature to relieve stress. The cooledglass may be cut into wafers of a desired size, lapped, and polished.The finished glass is a holographic glass in which a Bragg grating, orother hologram, can be optically recorded.

FIG. 3 provides a table listing the constituents of example holographicglasses made according to the foregoing method, using combinations ofraw materials such as those provided in FIGS. 2A and 2B. As describedherein, the finished glass may contain arsenic. The arsenic may bepresent as one or more arsenic oxides, such as As₂O₃ and/or As₂O₅.

The table of FIG. 3 provides an example ratio of the weight of eachconstituent to the total weight of the finished glass, given as apercentage of the total weight of the finished glass. For example, sucha glass may contain 0.1375% by weight of one or more arsenic oxides. Thetable also provides relatively narrow and relatively broad tolerances byweight for each constituent. For example, it might be tolerable for sucha glass to contain anywhere from 0.05 to 0.2% by weight of one or morearsenic oxides. It might be tolerable for such a glass to containanywhere from 0.02 to 0.4% by weight of one or more arsenic oxides.

Such holographic glasses may be used in the formation of opticalelements. Such an optical element may include a Bragg grating, forexample, holographically recorded in a three-dimensional bulk of aholographic glass. Such an optical element may be referred to as avolume Bragg grating (or “VBG”) element. Properties of VBG elements,methods for making and using VBG elements, and optical systems employingVBG elements have been described and claimed in U.S. Pat. Nos. 7,125,632and 7,298,771, for example. The entire disclosure of each of U.S. Pat.Nos. 7,125,632 and 7,298,771 is incorporated herein by reference.

Methods by which photo-induced images or phase holograms, for example,are recorded in a three-dimensional bulk of holographic glass generallyinclude two distinct steps. First, the bulk is exposed to light, wherebyphoto-induced changes of the material properties of the bulk areinitiated. Then, the bulk is subjected to thermal treatment, wherebychanges in color and refractive index of the material are completed.

Accordingly, two types of physical changes may occur in the holographicglass before a phase hologram is formed. First, a certain amount ofnano-clusters of metallic colloid phase may form in the glass. Second,small alkali-halide crystals of predominantly sodium fluoride may growon these nano-clusters to reach a combined volume sufficient to inducethe required refractive index changes in the exposed areas of glass.

The first type of physical change, i.e., the formation of nano-clusters(of metallic silver, preferably), manifests itself in the appearance ofcharacteristic absorption features, which may lead to yellow or tancolor hue of glass. FIG. 1 depicts an absorption band centered near 450nm.

The second type of physical changes, growth of alkali-halide crystalsinside the amorphous matrix of glass, may occur in the form ofnano-crystals scattered in the volume of glass. The difference in theindex of refraction in the exposed areas compared with unexposed areamay be determined by the total volume of a second phase formed insidethe glass matrix:

$\begin{matrix}{{\Delta\; n} = {\left( {n_{c} - n_{g}} \right)N\frac{4\pi}{3}{r^{3}(t)}}} & (1)\end{matrix}$where n_(c) is the refractive index of the crystalline phase, n_(g) isthe refractive index of the glass matrix, N is the concentration of thenano-crystals of the second phase, and r is the average radius of such acrystal. Crystals of non-spherical shapes may form.

The crystals of the second phase may also become scattering centers forthe light propagating inside the glass. The scattering loss coefficientα_(sc) produced by such a distribution of nano-clusters can be estimatedaccording to the Raleigh scattering formula:

$\begin{matrix}{\alpha_{sc} = {{N\;\sigma_{sc}} = {N \cdot {const} \cdot \frac{r^{6}(t)}{\lambda^{4}}}}} & (2)\end{matrix}$where σ_(sc) is the scattering cross section, and λ is the wavelength ofscattered light. There may be two distinct mechanisms of optical loss inthe holographic glasses: absorption by the metal nano-clusters (or metalcolloid), and scattering by the dispersed crystalline phase.Consequently, the process of a phase hologram formation may beaccompanied by an increase in loss (or a decrease in transparency) ofthe material.

To describe the performance of holographic glasses with recordedholograms for various wavelengths of test light, it is useful to use afigure of merit (FOM) that may describe the balance between the achievedcontrast of the recorded hologram (characterized by the induced changein the refractive index) and the accompanying optical loss in thematerial. The FOM may be defined as the ratio between the inducedrefractive index modulation and the total loss coefficient of thematerial at a specific wavelength:

$\begin{matrix}{{FOM} = \frac{\Delta\; n}{\alpha_{sc} + \alpha_{abs}}} & (3)\end{matrix}$where α_(sc) and α_(abs) are the scattering and absorption coefficientsof the material at a specific wavelength, respectively. Note that theFOM defined by equation (3) has the dimensionality of length, and may beexpressed in microns.

The FOM may be influenced by the amount of time that the holographicglass material spends in the thermal treatment cycle. Since at the earlystages of the thermal treatment cycle the absorption characteristic mayexist already but not the induced refractive index Δn, the FOM may startout small and increase up to a certain point. However, because thescattering coefficient α_(sc) grows significantly faster with the sizeof the crystallites than Δn (see Eqs. (1) and (2)), the FOM eventuallydeclines during the continued thermal treatment cycle. Therefore, theFOM should reach its maximum at some point during the growth of thehologram or VBG element. The maximum FOM is determined according to thetheory discussed above:

$\begin{matrix}{{FOM}_{\max} = {A \cdot \lambda^{2} \cdot \sqrt{\frac{ɛ(E)}{\sigma_{abs}\left( {\lambda,E} \right)}}}} & (4)\end{matrix}$where ε(E) is the fraction of colloidal particles that initiatenucleation of the crystalline phase, and σ_(abs)(λ, E) is thecharacteristic absorption cross section of the colloidal particles.Minimizing the absorption cross-section and maximizing the quantity ε(E)may be important means of improving transparency of phase holograms,particularly in the “soft” UV and visible spectral regions.

Trace element dopants that affect surface energy of metal colloidalparticles inside the glass matrix may lead to the dissolution of thesenano-clusters during the thermal treatment cycle and, therefore,increase the FOM of glass by reducing the effective absorption crosssection (Eq. 4). Therefore, any elements that are likely to interactwith the metal colloid particles and alter their surface energy canpotentially improve the FOM of holographic glasses. It has been found,for example, that the addition of arsenic to the glass composition showsevidence of such behavior (see FIG. 7).

In general, dopants that are likely to increase the FOM of holographicglass will typically exhibit some or all of the following properties.First, they may affect the oxidation/reduction behavior of silver orother colloid-forming metals (e.g., copper or gold) in the glass matrixduring and immediately after the light exposure and during the thermaltreatment cycle. They may facilitate formation of the nuclei of sodiumfluoride (or other alkali-halide crystals) on the colloidal particles ofsilver or other metals. They may facilitate dissolution of the metalcolloid at later stages of the thermal treatment cycle. And they mayhelp to control the size distribution of the metal colloid in the glassmatrix.

The addition of a dopant such as arsenic has been demonstrated to playan intimate role in the process of photo-induced phase hologramformation in such glasses. The data shows that significant improvementin the FOM of holographic glasses may occur upon addition of such adopant.

As is shown in Eq. (4), the maximum FOM of glass is determined by theabsorption cross-section of the metal colloidal particles and also bythe fraction of the number of these particles that become nucleationcenters of the crystalline phase. Note that Eq. (4) assumes that all theabsorbing centers survive throughout the thermal annealing process.Since absorption cross section of colloidal phase of silver, forexample, depends on the size and shape of the cluster, the position ofthe absorption maximum and the width of absorption peak are indicatorsof the change in the absorption cross-section. Such changes have notbeen seen to correspond to the presence or absence of trace elementdopants alone. Therefore, the primary explanation for the role of theseagents may be their affect on the fraction of the number of colloidalparticles that become nucleation centers of the crystalline phase. Asecondary explanation is their possible effect on dissolution of thecolloidal particles at later stages of thermal treatment when the nucleiof the crystalline phase have formed already. (See FIG. 7).

Hologram optical performance, and thus the FOM, may be influenced by thesize, shape, and volume concentration of the metal colloid inside theglass matrix, and also by the size and volume concentration of thenano-crystallites of the second phase. Control over these properties maybe exercised via a number of parameters: concentration of silver (orcopper, or gold) in the glass matrix, the oxidation level of the glass,the exposure to light, and concentrations of arsenic in the glassmatrix, for example. As a result, absorption features of the metalcolloid may be altered significantly. Nevertheless, it may be easy tounderstand that in order to serve as nucleation centers for the growthof the crystalline phase, the metal colloidal particles need to have acertain minimum size, and, in order to create sufficient index change inthe material, there should be certain minimum number of these clusters.

FIG. 4 provides plots of FOM for a prior art holographic glass madewithout arsenic, and a modified holographic glass that contains witharsenic. The FOM for the modified glass is shown for two wavelengths ofinterest: 473 and 532 nm. FIG. 4 also illustrates the FOM component Δnindex modulation for a holographic glass made with arsenic.

FIG. 5 provides a summary table of maximum FOM for holographic glassesmade with and without arsenic. The data in the table of FIG. 5 are givenfor three wavelengths of interest, which correspond to blue, green, andred light, respectively. As shown in both FIGS. 4 and 5, the FOM ofholographic glass made with arsenic may typically be significantlyhigher than the FOM of holographic glass made without arsenic asmeasured at any of the three wavelengths of interest.

FIG. 6 provides plots of total optical loss as a function of radiantexposure (i.e., the surface density of radiant energy received) for anincident beam having a wavelength of 473 nm. As shown, a prior artholographic glass made without arsenic tends to exhibit significantlymore total loss than a holographic glass that contains arsenic.

FIG. 7 depicts the absorption characteristic of a holographic glass madewith arsenic before and after final thermal treatment. As seen in FIG.7, the absorption may decrease, and, accordingly, the transparency mayincrease, after the final thermal treatment on a holographic glass. Thissuggests that the FOM of the glass may increase after final thermaltreatment.

As described herein, the addition of arsenic has been shown to improvethe FOM of holographic glasses, for incident light at certainwavelengths, over certain ranges of exposure. It should be understood,however, that the addition of other trace dopants may be found tosimilarly improve the FOM of holographic glasses. Such dopants may beidentified in accordance with the methods described herein. Further,though example glasses described herein were tested for exposures up toabout 1.0 J/cm², it should also be understood that holographic glassescontaining arsenic as described herein may exhibit higher figures ofmerit than comparable prior art holographic glasses without arsenic forexposures at any level. Similarly, though example glasses describedherein were tested at certain wavelengths, it should also be understoodthat holographic glasses containing arsenic as described herein mayexhibit higher figures of merit than comparable prior art holographicglasses at any wavelength.

What is claimed is:
 1. A photorefractive, holographic, silicate glasscomprising silver, bromine, fluorine, an oxide of cerium, and an oxideof arsenic, wherein the glass has a hologram recorded therein, whereinthe hologram is formed by changes in index of refraction within theglass, and wherein the glass has a figure of merit of at least sevenmicrons when measured for an incident beam of light having a wavelengthof 473 nanometers, wherein the figure of merit represents a ratio ofphotoinduced refractive index modulation to total loss coefficient. 2.The glass of claim 1, comprising at least 0.02 percent by weight of theoxide of arsenic.
 3. The glass of claim 1, comprising 0.02-0.4 percentby weight of the oxide of arsenic.
 4. The glass of claim 1, comprising0.05-0.2 percent by weight of the oxide of arsenic.
 5. The glass ofclaim 1, wherein the oxide of arsenic is As₂O₃.
 6. The glass of claim 1,wherein the hologram comprises a Bragg grating.
 7. The glass of claim 1,wherein the holographic glass is a photo-thermal-refractive glass.